Willow recruitment and channel patterns in beaver dominated stream systems

Abstract

<p>Beaver activity can segment a stream corridor through dam building which locally affects channel slope and sediment transport.  We have found, however, that long-term beaver presence within a river corridor has impacts beyond dam sites that affect system-wide stream morphodynamics and riparian willow recruitment. Along study streams (basin areas 20 – 125 km<sup>2</sup>) in southwestern Montana, USA, beaver-chewed willow stems (beaver cuttings) from dam construction, food caches and herbivory float downstream and commonly accumulate within 1 km of dam sites. At the 90 randomly selected sites surveyed, beaver cuttings accumulated on 81% of point bar sites and 51% of all surveyed sites. The accumulated beaver cuttings can sprout, adding roughness, thus enhancing sediment accumulation on point bars and at abandoned dam sites. Sprouting stems were present at 25% of all sites, indicating that beaver cuttings commonly provide a secondary pathway for willow recruitment and influence sediment dynamics.</p><p>As beaver cuttings and sediment accumulate on point bars, the channel migrates laterally, burying the cuttings. High resolution aerial imagery has been used to calculate migration rates for twenty-six 200 m reaches in the study streams. Migration rates range from 0.07 – 2.91 m/yr (mean 0.43 m/yr) over a 14 year period from 1995-2009. Thirty-four radiocarbon (<sup>14</sup>C) ages found in fluvial terraces 1.2 – 3 m above the bankfull channel, show that beaver cuttings range in age from ~6030 – 380 cal yr BP, demonstrating that deposition and burial of beaver cuttings on point bars has been a common process over millennia. The long-term preservation of beaver-chewed wood in point-bar sequences also attests to the importance of beaver activity for enhancing carbon storage in beaver-occupied stream systems.</p><p>The mosaic of sites created by beaver includes intact dams, recently breached or abandoned dams, and long-abandoned dams, interspersed with reaches unsuitable for beaver. The beaver produced habitat heterogeneity interacts with sediment and beaver cutting transport to enhance riparian plant colonization and meander development.</p>